Thesis
The effects of interfaces on heterogeneous nucleation of urea : experiments and simulations
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2025
- Thesis identifier
- T17185
- Person Identifier (Local)
- 202085076
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- The formation of crystals from solutions is a natural phenomenon, which is frequently used in industrial separation and purification processes. Crystal nucleation commonly occurs heterogeneously at interfaces, however, the processes are difficult to control since the behaviour is not well understood. This work uses a combination of nucleation experiments and atomistic molecular dynamics (MD) simulations to investigate the mechanisms influencing heterogeneous nucleation in aqueous urea solutions. Quiescent, cooling crystallisation experiments were performed and two high-throughput methods were compared to determine the optimal experimental conditions and setup. Concentrations of 1800-2280 g kg−1 and temperatures from -5-25 °C were tested, corresponding to moderate and high supersaturations. 15-20 °C provided the best induction time measurements with low nucleation probabilities at 25 °C. Heterogeneous nucleation experiments were performed for aqueous urea solutions in contact with air, glass, diamond and polytetrafluoroethylene (PTFE) surfaces. 1 ml solution samples were contained in glass vials with a solution-air interface, which was taken as the control system. The presence of a PTFE surface was found to significantly increase the nucleation probability, up to ×8, in comparison to the control samples, indicating that heterogeneous nucleation occurred at the PTFE surface. The presence of a diamond surface also showed a slight increase of nucleation probability, up to ×4. MD simulations were used to study urea solutions in contact with PTFE, diamond, α-quartz, and vacuum interfaces. Firstly, a force field validation protocol was developed, since the performance of MD simulations is highly dependent on the underlying models. Four GAFF and five OPLS force fields were compared, and a urea-specific charge-optimised GAFF force field and the original all-atom OPLS force field were found to be the best for simulating bulk crystals and aqueous solutions of urea. MD simulations of urea solutions at the solid interfaces found that strong solute-surface dispersion interactions enhanced the solute concentration near the interface. The interfacial urea concentration was enhanced by 32-59% and 13-29% for the PTFE and diamond surfaces respectively. The enhanced interfacial concentration contributes to the explanation for the observed heterogeneous nucleation rates and provides a new basis for guiding materials selection to improve crystallisation processes and to reduce fouling.
- Advisor / supervisor
- Mulheran, Paul
- Johnston, Karen
- Sefcik, Jan
- Resource Type
- DOI
- Date Created
- 2024
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